Boron, Sr, O, and H isotope geochemistry of groundwaters from Mt. Etna (Sicily)- hydrologic implications.

Combined B, O, H, and Sr isotopic studies of groundwaters from Mt. Etna provide new constraints on their origin and the consequences of fluid-rock interaction within this hydrologic system. Variations in ?18O (-9.7 to -7.2?) and ?D (-62 to -23?) mainly lie along the regional meteoric water line and suggest that most waters originated as local precipitation. However, small shifts in ?18O, and variable 87Sr/86Sr (0.70355 to 0.70879) and ?11B (-5.2 to +25.8?) indicate that subsequent interactions occurred between the fluids and local rocks. High B/Cl ratios in all samples seemingly preclude direct involvement of seawater in the hydrologic system despite the proximity to the coast and, in some samples, elevated 87Sr/86Sr and ?11B. Two general 'end-member' water types are recognized on the basis of their chemistry. These apparently are produced by interactions of local meteoric waters with the dominant reservoir rocks--either basaltic lavas of Etna or the underlying sediments, respectively; high ?11B in the sediment-hosted end-member points to a significant marine carbonate contribution. Subsequent mixing between these or similar end-members produced a range of intermediate composition groundwaters. Certain anomalous water compositions require the presence locally of a distinct component with high B and moderate ?11B (ca. +10?) but relatively low 87Sr/86Sr; an anthropogenic source for this component is plausible. One unusual sample has B and Sr isotopic compositions similar to the other volcanic rock-hosted waters, but anomalously high Cl content that likely reflects local magmatic outgassing near this sampling locality. In general, this study indicates that groundwater B and Sr isotopic compositions are rock-dominated; these data provide useful constraints on the origin and evolution of groundwaters.